1. Field of the Invention
The present invention relates generally to retroreflective sheets and, more particularly, to a retroreflective sheet having an adhesive surface that facilitates the reading of optical information from a remote distance.
2. Description of the Background
In order to automate a product distribution and administration system, bar codes and bar code readers are widely used. The bar code can indicate the contents and destination of a product by coding numerals thereon as a combination of bars of predetermined widths and having predetermined spaces therebetween. The bar code reader is used to optically read the bar code formed in such a manner.
The bar code may be a so-called "two-out-of-five" code that expresses values of 0 to 9 in the decimal notation by five binary bits, for example. A laser is the preferred light source for the bar code reader because of its many advantages, such as high light intensity, long life, and reading distance.
It has become desirable that the bar code be readable from greater distances. At the same time, it is desired that the bar code be so read without requiring modification to preexisting bar codes or to the conventional bar code readers.
To this end, it has been proposed that the light reflected from the bar code be concentrated in a predetermined direction to prevent scattering, thereby maximizing the amount of light that is focused onto the bar code reader.
For example, Japanese Utility Model Laid-Open Gazette No. 59-77101 describes a retroreflective sheet having the above-proposed property. In the manufacturing process of this retroreflective sheet, a metal foil is vapor-deposited onto a flexible base sheet such as cloth or paper, and a bar code is printed on the flexible base sheet. The bar code is composed of printing ink containing retroreflective glass beads, each bead having a refractive index greater than 2.
U.S. patent application Ser. No. 295,552 (Japanese Patent Application No. 63-39985) also proposes a bar code utilizing a retroreflective sheet. This previously proposed bar code utilizing the retroreflective sheet will be described hereinbelow.
The fundamental construction of such a retroreflective sheet is represented in FIG. 1. In FIG. 1, a plurality of very small balls or beads 11, each having a diameter of about 70 micrometers and made of glass or the like, are adhered to the entire surface of a base film 13 by means of a bonding layer 12 having a refractive index smaller than that of the glass beads 11.
If the wavelength of the incident light is constant and the refractive index of the glass bead 11 is properly selected, then the incident light can be substantially collimated with the reflected light, which includes the light refracted through the beads, reflected from the base film 13, and refracted again through the beads, and the light initially reflected from the surface of the beads 11. It has been found that the reflected, or more properly retroreflected, light from a retroreflective surface will have an intensity of more than 1000 times the intensity of the light reflected from a surface that does not contain retroreflective material, thereby increasing the distance at which such an improved bar code can be read.
U.S. patent application Ser. No. 295,552 discloses a positive-image bar code that utilizes the above-mentioned retroreflective material and is represented in FIG. 2.
As shown in FIG. 2, the bar code 10B is formed of a black base film 13BK on which reflecting portions, or bars 10a and dark portions, or spaces 10b are alternately aligned in accordance with a predetermined code. The positive-image bar code 10B may be modified to a negative-image bar code by inverting the bright reflecting portions and the dark portions.
Applying the above-proposed bar codes shown in FIG. 2 and in Japanese Utility Model Laid-Open Gazette No. 59-77101 to a preexisting physical distribution administration system requires that preexisting bar codes be replaced with new bar code patterns which must be printed and adhered to every product that is distributed according to the distribution system and then each new and existing bar code must be compared or checked on a per-unit basis, which is a laborious and time-consuming operation.